The present invention relates to a technology for detecting and analyzing with high sensitivity and accuracy, characteristic x-rays produced by bombarding a specimen with charged particles such as electrons in particular, or fluorescent x-rays produced by bombarding a specimen with x-rays.
For elementary analysis, there is a method by using characteristic x-rays or fluorescent x-rays produced by bombarding a specimen with charged particles such as electrons or x-rays, respectively.
This method makes use of the fact that characteristic x-rays or fluorescent x-rays emitted from a particular specimen have an energy peculiar to an element constituting the specimen. It is necessary for the elementary analysis that occurrence of the x-rays per unit time is counted for different energy of the x-rays. In general, semiconductor detecting elements made from semiconductor crystals such as Si or Ge are used for detection of x-rays.
Typical semiconductor detecting element are of the diode structure in which a pair of electrodes sandwich a semiconductor. When x-rays enter a depletion layer formed in the semiconductor by a voltage applied between the pair of electrodes in the reverse direction, secondary electrons are produced and generate electron-hole pairs proportional in number to the energy of the incident x-rays as the secondary electrons lose their energy. The generated electron and hole are attracted to the respective electrodes by the electric field between the electrodes and are extracted as a signal.
FIG. 4 shows a typical arrangement of a radiation detector employing the above semiconductor-type detecting element. X-rays 1 produced by bombarding a specimen 9 with electrons are detected by a detecting element 101. The detecting element 101 and a field effect transistor 2 at the input stage of a preamplifier 31 are cooled to a low temperature by a cryostat 7 to reduce noise.
The following explains processing of the signal obtained by the detecting elements. The electrons arriving at the electrode of the detecting element 101 are converted into voltage pulses 220 of heights proportional to the number of the electrons by a charge-sensitive preamplifier 31, and the voltage pulses 220 are filtered to increase its signal-to-noise ratio and shaped into voltage pulses 310 by a shaping amplifier 51. The voltage pulses 310 are analyzed in terms of their height to provide a pulse-height spectrum 400 of the x-rays.
The heights of the voltage pulses 310 corresponding to x-rays of a specific energy are not constant but varied due to the statistical fluctuation of the number of generated electron-hole pairs and noise in a pre-amplifier 31. Accordingly, a peak 501 in the spectrum 400 obtained by the pulse height analyzer 53 has a width of some value. The width 410 of the peak 501 at half its maximum height is called energy resolution. As the energy resolution is higher, i.e., the width of the peak is narrower, the signal-to-noise ratio becomes higher, the separation from peaks of other elements is improved, and the elementary analysis becomes more accurate.
A preamplifier circuit shown in FIG. 5 is of the light pulse feedback type described in IEEE Transaction on Nuclear Science, Vol. 18, No. 1, 1971, pp. 115-124, and FIG. 6 shows outputs of the preamplifier to specific incident x-rays.
In FIG. 5, the preamplifier circuit 31 comprises a field effect transistor 2 which works as, the first input stage, a light emitting diode 4, a feedback capacitor 6, an amplifying circuit element 8 which amplifies signals from the field effect transistor 2, and a comparator 55, and reference numeral 41 denotes a power supply.
Charges produced by incident x-rays 1 at the detecting elements 101 are stored in the feedback capacitor 6 and the output 220 at the preamplifier 31 build up stepwise as shown in FIG. 6. A succeeding circuit extracts steps 221 in the staircase waveform output 220 as x-ray signals. The output 220 increases stepwise for each incident x-ray and will rise close to a power supply voltage provided for this circuit and saturate eventually if no proper measures are taken.
In order to prevent the saturation of charges stored in the feedback capacitor 6, the following scheme is adopted. The comparator 55 generates a reset signal 223 when the level of output 220 reaches a predetermined value 222. By the reset signal 223, the light emitting diode 4 emits light 150 onto the field effect transistor 2. By this irradiation, a current can flow between its gate and source. The charges stored in the feedback capacitor 6 are discharged through this path and the level of the output 220 is returned to the initial state (reset). A high energy resolution is reported to be obtained by this method.
In addition to the above-described resetting method called light-pulse feedback, there are known a method employing a five-terminal field effect transistor as described in IEEE Transaction on Nuclear Science, Vol. 37, No. 2, 1990, pp. 452-456, pulse, and a method by taking advantage of impact ionization phenomenon as described in Nuclear Instruments and Physics Research, Vol. A378, 1996, pp. 583-588. They are different in a method of discharging a feedback capacitor, but they provide basically identical outputs.
A general-purpose shaping amplifier employs a semi-Gaussian filter for simplicity of construction, which is composed of an analog differentiator and plural stages of integrators as described in G. Bertolini, A. Coche, Semiconductor Detectors, North Holland, Amsterdam, 1968, pp. 232-236.
Among different types of filters, a cusp filter is ideal as described in Nuclear Instruments and Methods in Physics Research, Vol. A297, 1990, pp. 467-478, and the energy resolution by the use of the cusp filter is improved by 16% compared with that by the use of the semi-Gaussian filter. But cusp filters are difficult to realize and in actual practice, triangular filters are used which have intermediate performance between semi-Gaussian and cusp filters. Recently, to realize n ideal filter, filters of a digital signal processing type appeared. This type of filters convert outputs from a preamplifier into digital signals by an analog-to-digital converter and filter the signals in a digital domain as described in U.S. Pat. No. 5,349,193.
It is also effective for improving the sensitivity of the elementary analysis to receive x-rays from a specimen with an increased acceptance solid angle of the detector efficiently. The acceptance solid angle of the detector is inversely proportional to the square of a distance from the specimen to the detector and is proportional to the sensitive area of the detector. Accordingly, as the distance becomes shorter and the sensitive area becomes larger, the acceptance solid angle becomes larger. The distance is determined by considering damage of the detecting element and an available space.
The increase in the sensitive area deteriorates the energy resolution and increases difficulties in its fabrication, and the maximum practical sensitive area is several tens square millimeters. Therefore the acceptance solid angle of the detectors of the energy-dispersive x-ray spectroscopes for use in electron microscopes is in the range of 0.1 to 0.3 steradians at the most.
As another method of improving analyzing sensitivity, it is conceivable to increase a dosage of charged particles or x-rays to be irradiated onto the specimen and consequently to increase the amount of x-rays emitted from the specimen. But, in the above circuit, because of prevention of superimposition of pulses and the time required for conversion into pulse-height analysis, the maximum allowable amount of incident x-rays is several thousands per second and it is of no use to increase the dosage of x-ray radiation beyond the above maximum allowable amount. The dosage of x-rays onto specimens is also limited by damage caused to the specimens and it is desirable to minimize the dosage for an analysis of a biological specimen or a very small region of one nanometers square because they easily suffer serious damage.
In the above explanation, only one detecting element is used for the analysis. For the purpose of increasing the acceptance solid angle of the detector, Japanese Patent Application Laid-Open No. Hei 3-246862 discloses the arrangement of a pair of radiation detectors, and Japanese Patent Application Laid-Open No. Hei 9-92868 discloses the arrangement of plural detecting elements disposed in one container.
As a control system for a device including plural semiconductor detecting elements and plural charge-sensitive preamplifiers, Japanese Patent Application Laid-open No. Hei 3-26980 discloses a method which uses an additional circuit for comparing outputs of charge-sensitive preamplifiers each including a feedback capacitor and a feedback resistor with a reference voltage, and for discharging all the feedback capacitors if at least one of the outputs of the charge-sensitive preamplifiers reaches the reference voltage.
The above first method by arranging plural radiation detectors requires plural containers for housing the respective detecting elements and plural cryostats, and there arises a problem in that the volume occupied by the detectors increases, its operation flexibility is lowered, and its cost approximately doubles.
On the other hand, the above second method by housing plural detecting elements in one container does not consider influence by the interaction between signals from the different detecting elements. There is a problem in that, when one preamplifier is reset, another preamplifier receives noise, and consequently the energy resolution is degraded, or spurious peaks appear in the spectrum.
No consideration has been given to a problem in that the capacitance of the feedback capacitor which determines the gain of the preamplifier is influenced by its positional tolerances or its wiring, and leakage currents in the detecting elements differ from element to element, and consequently the output characteristics at the detecting elements and at the preamplifiers vary among the detecting elements and the preamplifiers, and the adjustment and correction for the variations between the outputs are complicated.
The above third method uses an additional circuit for comparing outputs of charge-sensitive preamplifiers with a reference voltage, and for discharging all the feedback capacitors if at least one of the outputs of the charge-sensitive preamplifiers reaches the reference voltage. This method is applicable to a preamplifier including a feedback capacitor and a feedback resistor in its feedback loop, but there has been a problem in that the energy resolution is inherently poor. No consideration has been given to the radiation detector having plural preamplifiers each provided with an independent reset circuit, to which the present invention is directed.
The following describes the results obtained by the present inventors for the arrangement of two detecting elements housed in one container and two prior art signal detection circuits.
FIG. 7 illustrates two x-rays of a manganese K .alpha. (MnK .alpha.) line (5.9 keV) and a manganese K.beta. (MnK.beta.) line (6.5 keV) in an x-ray spectrum produced by radiation from a sealed iron isotope having the mass number 55, which are often used for evaluating the energy resolution of an x-ray radiation detector in the low-energy region. Peaks 311 and 312 indicate MnKa and MnK.beta. lines respectively. A peak 313 was observed by a radiation detector employing one detecting element.
To investigate the cause for appearance of the peak 313, the out put wave forms of a pair of preamplifiers for the two detecting elements were studied. FIG. 8 shows an output 224 from one of the preamplifiers and an output 225 from the other of the amplifiers. Noise 227 was observed to ride on the output waveform 225 from the other amplifier when the waveform 224 from the one preamplifier is reset, and vice versa. It was found that the peak 313 was caused by the above noise. This noise is influenced by how internal electrical wirings are fabricated and where they are disposed. Noise appears as a peak having various heights at various positions and deteriorates energy resolution.
Although the combination of two detecting elements and signal detection circuits produces twice the amount of signal by a single detecting element, by using the sum of the spectrum from the two detecting elements in the identical measuring time, there is a problem in that a spurious peak is observed as described above, and consequently the analysis provides incorrect results or the measurement is not possible because of superimposition of a spurious peak on a peak produced by an element to be analyzed. Although it is conceivable to ignore noise caused by reset by distinguishing the noise from intended signals by digital signal processing, there is a problem in that a circuit for that processing is complicated and is costly.